Ethidium bromide, acridine dye, benzofuran, dibenzo-p-dioxane and the like conventionally used for staining cells and chromosomal genes are known as DNA intercalators that give rise to intercalation into double strand DNA molecules to form stable intermolecular bonds. For example, the formation of a strong intermolecular bond with genetic DNA by a DNA intercalator may cause inhibitory action on the expression of the genetic DNA. Many of these compounds that induce DNA intercalation are harmful to human bodies and their carcinogenicity is often pointed out. Although the mechanism of carcinogenesis resulting from DNA intercalators remains somewhat unclear, it is suggested that their properties of specifically interacting with double strand DNA are closely related to the mechanism.
Of DNA intercalators, polychlorinated dibenzo-para-dioxanes, polychlorinated dibenzofurans, coplanar polychlorinated biphenyls (hereinafter, referred to as dioxin and dioxin-like substances) are contained in trace amounts in exhaust from incinerators, industrial waste water, smoke from open-air burning and so on and have been formerly discharged into the environments such as air, soils and rivers. Dioxin and dioxin-like substances at low concentrations discharged into the environment are gradually condensed in the food-chain process of the natural world. As a result, the possibility of bringing about the accumulation of the dioxin and dioxin-like substances in the bodies of humans as final predators has been pointed out. In recent years, progress in measurement techniques allowed the quantification of trance amounts of dioxin and dioxin-like substances and made possible to estimate quantitatively the extent of pollution in the environments such as air, soils, groundwater and rivers and in foods, human bodies, breast milk and so on. Consequently, it has been found out that extensive pollution is taking place.
Because dioxin and dioxin-like substances are neither metabolized nor decomposed in organisms and are accumulated in their bodies, the dioxin and dioxin-like substances even at very low concentrations may have deleterious effects on human health, such as carcinogenicity, immunotoxicity and reproductive toxicity, posing serious social problems. Measures such as improvement in incinerators from which dioxin and dioxin-like substances are emitted, the prevention of discharge and the prohibition of open-air burning are taken to prevent dioxin and dioxin-like substance from being newly discharged into the environment. However, there is a demand for the removal and recovery of dioxin and dioxin-like substances that have already been discharged into the environment and are being formed inevitably.
Techniques for efficiently separating or decomposing and removing dioxin and dioxin-like substances at low concentrations are investigated in various quarters, for the purpose of removing and recovering dioxin and dioxin-like substances that have already been discharged into the environment or of reducing the discharge of dioxin and dioxin-like substances contained in exhaust and washing effluent from incinerators. For example, adsorption treatment with activated carbon, oxidative decomposition treatment with ultraviolet rays, ozone, hydrogen peroxide and so on, incineration treatment by decomposition at high temperatures and coagulation treatment by coagulation sedimentation have been proposed.
The activated carbon treatment is an approach that employs activation carbon exhibiting high ablility to adsorb various substances present in the dioxin and dioxin-like substances present in the gaseous phase. In this case, various other adsorbent molecules coexist with dioxin and dioxin-like substances at low concentrations and the adsorption activity of activated carbon is consumed by the adsorption of these coexisting adsorbed molecules. Accordingly, the adsorption activity of activated carbon decreases with the passage of time. The rate of removal of dioxin and dioxin-like substances is not always sufficient in light of time average. Moreover, activated carbon must be replaced periodically for maintaining its adsorption activity. This presents cost problems in systems that are continuously used. In the oxidative decomposition treatment, the oxidation of dioxin and dioxin-like substances in the gaseous phase is promoted under ultraviolet irradiation or by the action of ozone at high concentrations. However, its reactor is complicated and expensive. The incineration treatment allows oxidative decomposition in combustion atmospheres by maintaining temperatures around 1000° C. and thus requires large energy relative to very low throughput.
On the other hand, the coagulation sedimentation method seems to be possible means in treating polluted water or the like containing dioxin and dioxin-like substances at low concentrations. However, coagulation sedimentation in itself is a process that gradually progresses and requires space such as sedimentation basins having large capacities. Moreover, the coagulation sedimentation method utilizes, for example, the adsorption of dioxin and dioxin-like substances onto the surfaces of coagulating agents. As in the activated carbon treatment described above, the rate of removal of dioxin and dioxin-like substances is not always sufficient in light of time average.
Some approaches for overcoming the disadvantages of the conventional treatment methods have already been proposed.
Some approaches have been proposed for treating, at high removal rates, dioxin and dioxin-like substances dissolved at low concentrations in polluted water. One example thereof is a method in which a thin film of DNA insolubilized by irradiating, with ultraviolet rays, a film containing an aqueous solution of DNA or DNA generated from the aqueous solution is used as an adsorbent for dioxin and dioxin-like substances (see Japanese Patent Laid-Open No. 2001-081098). The method has an advantage that dioxin and dioxin-like substances in an aqueous solution is selectively bound to DNA contained in this thin film of insolubilized DNA to thereby allow the high-efficiency removal of the dioxin and dioxin-like substances at low concentrations in an aqueous solution. Specifically, the thin film of DNA is provided on a substrate such as a glass or plastic substrate, which is in turn irradiated with ultraviolet rays with a particular wavelength to thereby produce a complex having the film of DNA insolubilized and immobilized on the substrate. Effluent containing dioxin and dioxin-like substances is allowed to flow into treatment equipment in which this complex having the DNA film is placed. On contact of the effluent with the complex having the DNA film, the dioxin and dioxin-like substances are selectively adsorbed onto this DNA film that is insolubilized and immobilized. Thereby, the dioxin and dioxin-like substances at low concentrations are selectively removed with high efficiency.
However, because the method described above employs ultraviolet irradiation to insolubilize and immobilize DNA on the surface of a substrate, the surface area of the DNA film that adsorbs dioxin and dioxin-like substances relies on the surface area of the substrate. When the method is applied to treatment for large amounts of polluted water, complexes having DNA films must be used in large amounts for enlarging the adsorptive surface area of the DNA film. Thus, the preparation of large amounts of complexes having DNA films in advance is a major technical obstacle in applying the method to treatment for large amounts of polluted water. On the other hand, single strand DNA and double strand DNA differ in the property of adsorbing dioxin and dioxin-like substances. In addition, the length and molecular weight of DNA strands should also affect the property of adsorbing dioxin and dioxin-like substances. Japanese Patent Laid-Open No. 2001-081098 does not sufficiently describe means and conditions for optimizing the property of adsorbing dioxin and dioxin-like substances in the thin film of DNA insolubilized by ultraviolet irradiation.
In addition to the approach employing the film of DNA that is insolubilized and immobilized, a technique for large-scale and efficient removal for the purpose of treating larger amounts of polluted water has also been proposed (see Norio Nishi et. al., High Polymer Japan 52, 134 (2003)). In this approach, which uses an aqueous solution of double strand DNA trapped inside of a dialysis membrane, effluent containing dioxin and dioxin-like substances is allowed to flow on the outside of the dialysis membrane and the dioxin and dioxin-like substances contained in the effluent are diffused across the dialysis membrane, and accumulated and condensed in the aqueous DNA solution. The aqueous solution of double strand DNA having the accumulated dioxin and dioxin-like substances is then washed with an organic solvent that can dissolve the dioxin and dioxin-like substances to recover the dioxin and dioxin-like substances into the organic phase, and the washed aqueous solution of double strand DNA is repeatedly utilized. In this method, the contamination of microorganisms into an aqueous solution of DNA used in the treatment sometimes renders the aqueous solution of DNA rotten. Moreover, the stability of double strand DNA becomes a problem in long-term use.